Which function replicates elements of vectors?

Which level plotting commands generate figures? 

How to check if an R object is NULL?

The ______ function cross-tabulates tables using formulas.

 We can dump() R objects to a file by passing _____ 

Which of the following extracts the first element from the R vector below?

x <- c("a", "b", "c", "c", "d", "a")

A function that returns a vector of the same size as x with the elements arranged in increasing order. 

Setting the random number seed with _____ ensures reproducibility of the sequence of random numbers.

Code to generate a sequence of integers from 1 to 10

Which of the following R code can be used to avoid numeric problems such as taking the logarithm of a non-positive number?